Large generators are driven by a prime mover to produce a supply of electric energy. A generator rotor is energized by an exciter providing to it a supply of direct current (DC) power effective to produce a magnetic field rotating with the rotor. An annular stator surrounding the rotor contains a plurality of windings in which electricity is induced by the rotating magnetic field.
Providing the supply of DC power to the rotor involves transferring the DC power from a stationary element to the rotating element. One method of transferring the DC power includes slip rings rotating with the rotor and stationary brushes contacting the slip rings.
Slip-ring techniques are subject to reliability problems. An improved technique for transferring power from the stationary to the rotating element is conventionally known as a brushless exciter in which a DC field is applied to a stationary exciter winding. One or more windings rotating with the rotor pass through the magnetic field produced by the stationary exciter winding thereby producing alternating current (AC) power. The exciter AC power is rectified in a rectifier located on the rotor to produce the required DC excitation.
The amount of DC exciter power required by the rotor varies with the generator load. That is, as the generator load increases, the magnitude of the rotor magnetic field must be increased to maintain the desired output. This is conventionally accomplished by varying the amount of DC power fed to the stationary exciter winding. The DC power may be controlled by a control signal or in response to a measurement of the generator output voltage, optionally combined with a measurement of the generator output current.
An exciter field control responds to a drop in the generator output voltage with an increase in DC voltage fed to the stationary exciter winding. In some installations, the magnitudes and phase relationships of the output voltage and current are employed to compensate the exciter field voltage for the reactive component of the generator output.
For reference to the details of a conventional brushless exciter as described above, reference is hereby made to U.S. Pat. No. 4,723,106 (Gibbs, et al.). Conventional brushless exciters as described above typically include a rotating rectifier bridge with a plurality of rectifier assemblies, with each rectifier assembly having a number of parts which must be assembled together on the inside of a diode wheel rim. The various parts are usually held in place on the inside of the diode wheel rim by hand or with a wedge. A clamping bolt extending through the diode wheel rim from the outside of the diode wheel rim passes through an outer heat sink and an inner heat sink to a clamping bar located adjacent the inner heat sink. A problem with assembling a rectifier assembly in this manner is that the various parts of the rectifier assembly must be manually assembled within the diode wheel rim. Space is limited within the diode wheel rim, making assembly sometimes difficult. Moreover, visual inspection of the rectifier assembly within the diode wheel rim is difficult after assembly inside the diode wheel rim. Thus, it is nearly impossible to ensure that the rectifier diodes have been properly seated between the inner heat sink and the outer heat sink, which is necessary to ensure both good electrical as well as thermal conductivity.
The present invention is directed to overcoming one or more of the problems as set forth above.